GOOGLE the following article from the Huffington post, and see the vulnerable seats by name :

2014 Senate Races Likely To Keep

Democrats On The Defensive ...

" Twenty Democratic senators are up for reelection in 2014, compared to
only 13 Republicans. Despite the Democratic successes the last time
this group of senators was up for election in 2008, many of their seats
are in Republican-leaning, or swing, states.

An initial analysis of the 2014 Senate races -- primarily based on the
partisan leanings of each state and the perceived strength of the
incumbents -- shows 12 Democratic seats that are potentially in play,
compared with just two Republican seats that could be vulnerable,
ensuring that Democrats will almost exclusively be playing defense in
2014. "